I can't say that I went into that much depth when I went through reviews to choose the one I got. I do read them, not just count the stars, and I tend to read both the 5 stars and the 1 stars to see what the general thoughts or specific problems might be. Nothing really stood out to me on these adapters though.
To my mind they're pretty straightforward: Power plug and cord (hard to mess that up) go into a AC to DC converter (pretty hard to screw that up too), then DC voltage at the amperage rating required for the unit comes out through another cable (not much mystery there) to a dummy battery that fits the camera compartment and lines up the contacts correctly (again, not hard to achieve). I've spent a lifetime working with electronics, building and repairing circuits, etc., though, so that might bias me on this.
Reviews are kind of a funny thing. You can choose your best piece of gear, and find reviews from people who will swear up and down that it's the worst piece of crap they ever dealt with, and that you should avoid it at all costs. And, I think you mentioned, that sometimes it could very well be the competition rating it down, or even the manufacturer surreptitiously rating it up, which just skews things even more.
Sometimes, it seems that all you can really do is go by the fact that 95% of people who've bought and used something are satisfied with it, and only a small percentage aren't, perhaps just because it was damaged in shipping or user error, or they bought the wrong one and it doesn't fit their needs or got shipped the wrong one, or whatever.
In any case, the cheapo one I bought does seem to work fine for me, every time I use it, and it was dirt cheap, especially compared to the cost of the genuine Canon adapter.
Are there any manufacturers/models in particular that were so bad from the reviews you read that you would warn folks specifically against them?